1. Field of the invention
The present invention is directed to a simple low cost hydrocarbon vapor sensor and system employing the same. The sensor is used, for example, for detecting hydrocarbon leaks in buried fuel storage tanks. The vapor sensor can employ carbon-filled ethylene propylene rubber (EPR) semiconductive tape as a sensing element or a normally closed switching elastomer that opens when hydrocarbon vapor is adsorbed.
2. Description of the Related Art
Monitoring fuel storage tanks, particularly those underground, for hydrocarbon leaks is an exceedingly important environmental concern. Current detection/monitoring systems for monitoring leaks in fuel storage tanks can employ, for example, semiconductor, capacitive, and conductive liquid crystalline sensors or gas analyzers for detecting liquid or vapor leaks. These systems are complicated and very expensive.
Electrically conductive polymeric materials, such as conductive rubber, have been used for detecting liquid hydrocarbons, but must be placed at locations such as a sump where leaking liquid will collect and directly contact the sensor.
U.S. Pat. No. 4,631,952 to Donaghey, teaches a prior art device that employs a resistive sensing element which is sensitive to vapor as well as liquid. The sensing element includes an admixture of a swellable matrix and conductive particles, such as synthetic rubber, polyvinyl chloride, silicone rubber and finely divided carbon. Silicone rubber is preferred. The sensor is sensitive to five parts per million (ppm) of hydrocarbon vapor. This sensor, however, does not provide for temperature compensation. Therefore, unless it is in an environment in which the temperature is constant, resistance changes due to even small temperature changes will exceed those due to vapor concentration. Consequently, devices such as those taught by Donaghey are not practical.
Another prior art sensor is a silicone polymer sensor. Such sensors conduct electricity but are not affected by water. The sensor is sensitive to liquid hydrocarbon. The sensor has a low resistance and has a high density of carbon black particles. This type of sensor is not responsive to gas-phase hydrocarbons.
Additional prior art sensors include combustion energy, flame ionization, gas chromatography, chemical, absorption force, and optical sensors. These sensors are all expensive, complicated and are not suitable for detecting hydrocarbon leaks.